Your fuel pump switch is opening and closing. On some models the AFM flapper has a switch and as the car idles down the pump shuts off, pressure drops, the mix gets right it fires, the AFM flapper moves the pump comes back on, goes rich from high fuel pressure and the cycle repeats.
The later cars use an oil pressure switch which wasn't supposed to do that, but...
Like SleeperZ suggests, find out if your pump is turning on and off. With 40psi you may also have an obstruction in your return line---blow through it to check if it's clear.
Once you know what the pump I scoring, you then just need to see why, which is straightforward, not much in the system.
Use the CORRECT FSM DIAGRAM! Like i said, the control circuit changed in 76 I think, so early books won't reflect way is in the later two years of production. Reading the right manual saves a lot of headaches!!!